US 8000427 B2 Abstract Provided is an apparatus and method for low-complexity scheduling in a multi-user Multi-Input Multi-Output (MIMO) system. In a method for scheduling in a MIMO system, a vector is set on the basis of channel information for all user terminals. A scalar for the vector is initialized. A user terminal that has the greatest scalar among unselected user terminals is selected. A user set is updated by adding the selected user terminal therein. The vector and the scalar for a user terminal not included in the user set are updated. Accordingly, a high transmission capacity can be provided while minimizing the influence of an interference signal. Also, the computation amount can be reduced greatly.
Claims(14) 1. A method for scheduling in a Multi-Input Multi-Output (MIMO) system, comprising the steps of:
setting a vector, for each of a plurality of user terminals, based on channel state information from a respective user terminal, and computing a scalar, for each of the plurality of user terminals, from a respective vector, wherein the scalar indicates a channel state of the respective user terminal;
selecting a user terminal that has a greatest scalar among unselected user terminals;
updating a user set by adding the selected user terminal therein; and
updating a vector and a scalar for each user terminal not included in the user set such that an attribute of a preselected user terminal is removed.
2. The method of
3. The method of
_{k }is set as
b _{k}=h_{k } where h
_{k }is a (1×N) vector of complex numbers that indicates a channel between a k^{th }user terminal and a transmitter and k denotes a search parameter for 1 through K user terminals.4. The method of
_{k }is set as
b _{k}=√{square root over (ρ_{k})}h_{k } where b
_{k }denotes a vector of a k^{th }user terminal, h_{k }is a (1×N) vector of complex numbers that indicates a channel between the k^{th }user terminal and a transmitter, ρ_{k }denotes channel quality information, and k denotes a search parameter for 1 through K user terminals.5. The method of
_{k }is initialized as
p _{k}=∥b_{k}∥^{2 } where b
_{k }denotes a vector of a k^{th }user terminal, the scalar p_{k }is a value indicating the channel state of each user terminal, which is used as a condition value for a scheduling process, and k denotes a search parameter for 1 through K user terminals.6. The method of
7. The method of
8. The method of
9. The method of
b _{u} _{ n } =b _{u} _{ n }/√{square root over (p_{u} _{ n })}where u
_{n }denotes a selected user terminal, b_{u} _{ n }denotes a vector of the selected user terminal u_{n}, and p_{u} _{ n }a scalar of the selected user terminal u_{n}.10. The method of
b _{k} =b _{k}−(h _{k} b _{u} _{ n−1 }*)b _{u} _{ n−1 } p _{k}=b_{k}h_{k}*where b
_{k }denotes a vector of the k^{th }user terminal, p_{k }denotes a scalar of the k^{th }user terminal, h_{k }is a (1×N) vector of complex numbers that indicates a channel between the k^{th }user terminal and a transmitter, * denotes a complex transpose, k denotes a search parameter for 1 through K user terminals, and u_{n−1 }denotes the preselected user terminal.11. An apparatus for scheduling in a Multi-Input Multi-Output (MIMO) system, comprising:
a channel state feedback unit for estimating a channel state and feeding back channel state information; and
a multi-user scheduler for setting a vector, for each of a plurality of user terminals, based on channel state information from a respective user terminal, and computing a scalar, for each of the plurality of user terminals, from a respective vector, wherein the scalar indicates a channel state of the respective user terminal, selecting a user terminal that has the greatest scalar among unselected user terminals, updating a user set by adding the selected user terminal therein, and updating a vector and a scalar for each user terminal not included in the user set such that an attribute of a preselected user terminal is removed.
12. The apparatus of
13. The apparatus of
an information collector for outputting data of selected user terminals among all of the user terminals; and
a preprocessor for precoding data corresponding to the selected user terminals in a predefined scheme and transmitting resulting data to respective users through a predetermined number of TX antennas.
14. The apparatus of
Description This application claims priority under 35 U.S.C. §119(a) to an application filed in the Korean Intellectual Property Office on Aug. 10, 2006 and assigned Serial No. 2006-75582, the contents of which are incorporated herein by reference. 1. Field of the Invention The present invention relates generally to a Multi-Input Multi-Output (MIMO) system, and in particular, to an apparatus and method for low-complexity scheduling in a multi-user MIMO system. 2. Description of the Related Art It is known that a MIMO channel can provide very high transmission efficiency, especially in multi-user environments. In the multi-user environments, a sum capacity (i.e., a maximum sum of transmission rates) can be achieved by allocating the same resource to a plurality of users simultaneously. The terms ‘user’ and ‘user terminal’ will be interchangeably used herein. In a case of a system using N transmission (hereinafter, “TX”) antennas, the sum capacity (i.e., the theoretical maximum transmission capacity) can be approached by allocating the same resource to up to N users simultaneously in consideration of the complexity of a transmitter. However, the selection of N users for maximization of the transmission capacity is not practically viable because the selection requires a very high complexity. Several techniques have been proposed to suppress a multi-user interference for the maximization of the transmission capacity. For minimization of the multi-user interference, the above conventional techniques select users, one-by-one, on the basis of channel information fed back from all of the users, which necessitates performing a large amount of computation at a corresponding transmitter. The amount of computation for multi-user selection is determined according to the number of TX antennas and the number of users that will receive data simultaneously. The conventional techniques can reduce the computation amount in linear proportion to the number of users. However, the conventional techniques are not practically viable in real time at a cellular base station because they still require the computation amount corresponding to the fifth-order term of the number of TX antennas. Hereinafter, a description will be given of a conventional scheduling technique based on a successive projection scheme, assuming that there are K users within the coverage of a serving base station with N TX antennas. The conventional scheduling technique selects a user of a channel that is least correlated with a channel of previously-selected users, i.e., the k At the k Assuming that H The above process is repeated until N users, which will receive data simultaneously, are selected among the K users. However, a matrix product and an inverse matrix must be computed in order to determine one k An aspect of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide an apparatus and method for low-complexity scheduling in a multi-user MIMO system. Another aspect of the present invention is to provide an apparatus and method for low-complexity scheduling in a multi-user MIMO system, which can provide both a high transmission capacity and a low complexity by selecting a set of user terminals with the highest transmission capacity among multi-antenna users. According to one aspect of the present invention, a method for scheduling in a MIMO system includes setting a vector on a basis of channel information for all user terminals and initializing a scalar for the vector; selecting a user terminal that has a greatest scalar among unselected user terminals; updating a user set by adding the selected user terminal therein; and updating the vector and the scalar for a user terminal not included in the user set. According to another aspect of the present invention, an apparatus for scheduling in a MIMO system a channel state feedback unit for estimating a channel state and feeding back channel state information; and a multi-user scheduler for setting a vector on a basis of channel information for all user terminals, initializing a scalar for the vector, selecting a user terminal that has a greatest scalar among unselected user terminals, updating a user set by adding the selected user terminal therein, and updating a vector and a scalar for a user terminal not included in the user set. The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. Hereinafter, a description will be given of an apparatus and method for low-complexity scheduling in a multi-user MIMO system according to the present invention. Referring to Using channel state information received from the respective user terminals, the multi-user scheduler The information collector In order to transmit data to the selected users simultaneously, the preprocessor The receiver For example, a matched filter, a zero-forcing technique, or a Minimum Mean-Square Error (MMSE) technique may be used to extract data from signal received by the receiver The channel state feedback unit Referring to In order to select N user terminals by a scheduling process, the multi-user scheduler In step If the n is less than or equal to the N, in step The user terminal u
In step The update of the user set S and the normalization of the channel vector b of the selected user terminal u In step If the k is not included in the selected user set S, in step The b Thereafter, the multi-user scheduler On the other hand, if the k is included in the selected user set S, in step On the other hand, if the k is greater than or equal to the K (in step On the other hand, if the n is greater than the N (in step Meanwhile, the present invention can extend or change a scheduling scheme by setting the initial value of the vector b In a case of a subspace-based scheduling scheme, the vector b In a case where the user terminal with multiple antennas uses only one or more of the multiple antennas, the vector b Lastly, a plurality of transmission channels may be allocated to one user by allocating a plurality of vectors b Referring to The computation amount of the proposed algorithm and the computation amount of the conventional algorithm are compared in Table 1 below.
In Table 1, N denotes the number of TX antennas of a base station and K denotes the total number of user terminals in a sector of a cell. For example, when N is 4 and K is 100, the computation amount can be expressed as Table 2 below.
It can be seen from the above that the algorithm of the present invention can greatly reduce the computation amount when compared to the conventional algorithm. As described above, according to the present invention, the base station can obtain a set of user terminals with the highest orthogonality by sequentially selecting user terminals that have the highest transmission capacity among multi-antenna user terminals in the multi-user MIMO system. Also, by transmitting data of the selected user terminals, the base station can provide a high transmission capacity while minimizing the influence of an interference signal. Also, the computation amount can be greatly reduced because there is no need to compute an inverse matrix. While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Patent Citations
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